1. Field of the Invention
The invention concerns an illumination system for wavelengths of ≦193 nm with a light source that provides a predetermined illumination A in a plane. The illumination system comprises a first optical component with first raster elements, which are arranged on a support element, wherein the first optical component is disposed in, or in the vicinity of, the plane in which the illumination is provided. The wavelength preferably lies in the wavelength range of ≦100 nm, particularly in the wavelength range which can be used for EUV lithography, i.e., in the range of 11 to 14 nm, in particular, at 13.5 nm.
2. Description of the Related Art
In order to still further reduce the structural widths for electronic components, in particular, to the submicron range, it is necessary to reduce the wavelengths of the light utilized for microlithography. The use of light with wavelengths smaller than 100 nm is conceivable, for example, lithography with soft x-rays, so-called EUV lithography.
EUV lithography is one of the most promising future lithography techniques. At the present time, wavelengths in the range of 11-14 nm, in particular, 13.5 nm, are discussed as wavelengths for EUV lithography, with a numerical aperture of 0.2 -0.3. The image quality in EUV lithography is determined, on the one hand, by the projection objective, and, on the other hand, by the illumination system. The illumination system will provide an illumination that is as uniform as possible of the field plane, in which the pattern-bearing mask, the so-called reticle, is disposed. The projection objective images the field plane into an image plane, the so-called wafer plane, in which a light-sensitive object is disposed. Projection exposure systems for EUV lithography are designed with reflective optical elements. The shape of the field in the image plane of an EUV projection exposure system is typically that of an annular field with a high aspect ratio of 2 mm (width)×22-26 mm (arc length). The projection systems are usually operated in scanning mode. With respect to EUV projection exposure systems, reference is made to the following publications:
W. Ulrich, S. Beiersdorfer, H. J. Mann, “Trends in Optical Design of Projection Lenses for UV- and EUV-Lithography” in Soft-X-Ray and EUV Imaging Systems, W. M. Kaiser, R. H. Stulen (Eds.), Proceedings of SPIE, Vol. 4146 (2000), pages 13-24
and
M. Antoni, W. Singer, J. Schultz, J. Wangler, I. Escudero-Sanz, B. Kruizinga, “Illumination Optics Design for EUV-Lithography” in Soft X Ray and EUV Imaging Systems, W. M. Kaiser, R. H. Stulen (Eds.), Proceedings of SPIE, Vol. 4146 (2000), pages 25-34,
the disclosure content of which is incorporated to the full extent in the present application.
In order to image a mask disposed in an image plane of the illumination system onto a light-sensitive substrate, for example, a wafer, which can be used for the production of semiconductor components, it is necessary that the form of the illumination and the lighting intensity are kept constant in the image plane, in which the mask is disposed, during the exposure process. A change in the illumination can be caused both by the power as well as by position due to a degradation of the source or a change in the position of the source.
In particular, in illumination systems that consist of two subsystems, a first subsystem, which comprises a light source and first optical elements that image the light source, for example, into a real intermediate image, and a second subsystem, which comprises second optical elements in order to illuminate a field in a field plane, for example, by means of a change in the radiating characteristic of the source, the position of the source or even a decentering or misalignment of the first and second subsystems, the illumination of a field can fluctuate in a field plane, or a loss of uniformity in the field plane as well as telecentricity error in the field plane can arise.
In EP 0 987,601 A2, a projection exposure system is described, which comprises a device, with which the intensity of an illumination, the so-called exposure dose, can be determined. In addition, an illumination system is indicated in EP 0 987,601 A2, which has a first subsystem, comprising a light source and first optical elements, wherein the first subsystem images the light source into an intermediate image. In addition, the illumination system comprises a second subsystem, which is disposed downstream from the intermediate image of the light source in the light path from the light source to the field plane, whereby the second subsystem comprises second optical elements for illuminating a field in a field plane.
In addition, in EP 0 987,601 A2, it is generally described that, for aligning the first and second subsystems, the illumination device comprises a detection device, with which positional deviations of the optical axis between the first subsystem, the so-called source system, and the second subsystem are detected. For this purpose, EP 0 987,601 A2 in general proposes to arrange sensors on a facet mirror disposed in the second subsystem. In order to be able to align the optical axis as a function of the detected deviation, it is described in EP 0 987,601 A2 to design the collector mirror in a positionable manner, so that it is possible to correct a misalignment of the first and second subsystems by adjusting the collector mirror.
It is a disadvantage in the system according to EP 0 987,601 that a misalignment or decentering of the optical axis of the illumination system is detected by means of the change in the photoelectric effect, which occurs on one or both facetted optical elements of the illumination system. The change in the photoelectric effect is caused by all raster elements or facets of the facetted elements. This determination is very complicated.
A further disadvantage of the system described in EP 0 987,601 is that an insufficiently precise detection of the illumination is achieved in the field plane. Essentially only a decentering of the optical axis is detected in an illumination system consisting of two subsystems, wherein the first subsystem represents a source unit and the second subsystem comprises optical elements for illuminating a field plane.
Another disadvantage of the device of EP 0 987,601 A2 is that the number of photoelectrically generated electrons is dependent on the degree of cleanliness of the surface. Since mirror surfaces become contaminated to some extent with long operating times, a large change in output signals must be taken into consideration during operation. In addition, a plurality of light wavelengths generally strike the first mirror, in particular, which is provided with a multilayer coating, so that the measurement results are falsified as a result of the different spectral distributions.
The correction of the decentering of the first subsystem relative to the second subsystem by means of the collector has the disadvantage that, since the collector only makes possible a stigmatic imaging on a well-defined axis, the collector axis, a change of the collector position leads to aberrations. Further, the collector, which is connected, for example, to a cooling unit, comprises a mechanically large and heavy structural unit, which can be manipulated only in a complex manner.